This study is an attempt to measure religious fertility differentials of the Abangan and the Santri, two different Muslim groups on Java. The Santri are those who strongly endeavor to carry out the religious prescriptions and observances; the Abangan, on the other hand, are individuals whose religious orientation is not as strong. The main purpose of the study is to determine if religion explains the high fertility level on Java. The results are expected to be important for the population policy makers enabling them to determine the factors influencing fertility level in the area studied.Studies of the literature lead to the formation of the theory employed for the study, which is a hybrid: drawing elements from four theoretical explanations of religious differences in fertility. It maintains that religious doctrine affiliation interacts with socioeconomic factors in explaining fertility. Three stages of socioeconomic level are assumed: (1) traditional; (2) transitional; and (3) modern. The study hypothesizes that religious fertility differentials of the two groups are slight when they are in the traditional stage. Further, the Santri are predicted to maintain higher fertility level than the Abangan.The data accessed for the study are from a 1974-1975 study on transmigration on Java. The sampling procedure was a stratified probability random sampling, with 3000 individuals of rural and urban areas on Java. One major weakness encountered in the data is the lack of detailed information on fertility determinants such as contraceptive use, marriage duration, and age of wife at first marriage.Among other things, one factor that justifies the use of the data is the extensive information religious practices of the people.Multiple regression analyses are conducted to test the hypotheses of the study. The data is analyzed by computer using the SPSS (Statistical Package for Social Sciences) program. The significant test employed to measure the impact of the independent variables on the dependent variables is the F test.Taking into consideration that the majority of the respondents are from a low socioeconomic level and rural residents, the stage being observed in this study is the traditional one. Two measures are employed to determine fertility, namely, number of children ever born and ideal family size preferred by married couple when they started the household.Factors employed to explain the fertility levels are religious group affiliation, education, wife's age, and income. The study found little significant difference on the cultural and in the fertility level of the two groups. However, with additional analysis it was found further that the interaction of religious doctrine affiliation and wife's education correlates significantly with the ideal family size preferred, but not with the number of children ever born.The insignificant difference of the fertility levels of the two groups is rather surprising. One possible explanation for the case is the discrepancy between the theoretical definition of the two religious groups and the empirical phenomena. As a matter of fact, the Muslim groups on Java are not as clearly distinguishable as suggested in this study. The discrepancy results in the inaccuracy of the data analysis, for those who in the study are considered as belong to the Abangan category are actually residual cases of the Santri category. Hence, analyses are not accurate. Had the two groups been measured based on the theoretical concepts, the results of the study could possibly be as expected.Although the study is limited in its intensity, the results are valuable for future studies. It is recommended that other factors such as cultural values, information pertaining to birth control , and family planning be considered for future studies. Specific measurement of religious groups studied would yield a better explanation on religious fertility differential.